Mapping the circumstellar SiO maser emission in R Leo

Context. The study of the innermost circumstellar layers around AGB stars is crucial to understand how these envelopes are formed and evolve. The SiO maser emission occurs at a few stellar radii from the central star, providing direct information on the stellar pulsation and on the chemical and physical properties of these regions. Our data also shed light on several aspects of the SiO maser pumping theory that are not well understood yet. Aims. We aim to determine the relative spatial distribution of the 43 GHz and 86 GHz SiO maser lines in the oxygen-rich evolved star R Leo. Methods. We have imaged with milliarcsecond resolution, by means of Very Long Baseline Interferometry, the 43 GHz (28 SiO v=1, 2 J=1–0 and 29 SiO v=0 J=1–0) and 86 GHz (28 SiO v=1 J=2–1 and 29 SiO v=0 J=2–1) masing regions. Results. We confirm previous results obtained in other oxygen-rich envelopes. In particular, when comparing the 43 GHz emitting regions, the 28 SiO v=2 J=1–0 transition is produced in an inner layer, slightly closer to the central star than the v=1 J=1–0. On the other hand, the 86 GHz 28 SiO v=1 J=2–1 line arises in a clearly farther shell. We have also mapped for the first time the 29 SiO v=0 J=1–0 emission in R Leo. The already reported discrepancy between the observed distributions of the different maser lines and the theoretical predictions is also found in R Leo.